In a developing multicellular organism, gradients of secreted signaling molecules coordinate growth and patterning along the body axes leading to the formation of specialized structures in precise locations. One such family of secreted signaling molecules, encoded by the Wnt genes, plays pivotal roles during both embryogenesis and tumorigenesis. Two different classes of Wnts have been identified, based on their activity in axis duplication and transformation assays. The primary focus of our research is to understand the molecular mechanisms by which these different classes of Wnts regulate the specification and patterning of the body axes during early mouse development. All the cells of the embryo proper arise from a sheet of pluripotent cells termed the epiblast. Gastrulation converts the epiblast into the three primary germ layers: the ectoderm, from which the skin and nervous system arise; the mesoderm, which generates the skeletal system and internal organs; and the endoderm, which gives rise to the gut and associated organs. Members of both classes of Wnts are co-expressed during gastrulation in patterns that suggest roles in germ layer formation and patterning. Targeted mutations generated in representatives of both classes of Wnts revealed that although Wnt8 is not essential, Wnt5a is required for proper gastrulation. Defects in the outgrowth of the embryonic trunk and tail are observed, and similar phenotypes are observed during the outgrowth of the face, tongue, limbs, external ear, external genitalia, and the gastrointestinal tract. The abnormal morphologies observed in these disparate structures bear a striking resemblance to each other, suggesting that Wnt5a plays a fundamental role in regulating tissue morphogenesis. Using a combination of embryological techniques (e.g., lineage tracing, transplantation, whole embryo electroporation and culture) coupled with molecular and cellular approaches, our current studies are directed towards understanding how Wnt5a may regulate stem cell proliferation. A second line of investigation is aimed at understanding the potential role of Wnt5a in regulating the cell-cell interactions that lead to tissue polarity and convergent-extension movements during gastrulation. We are taking both proteomics and microarray approaches to identify new targets of the Wnt5a pathway. In contrast to the Wnt5a mutants, embryos lacking Wnt3a completely lack posterior trunk and tail somites, forming ectopic neural structures at the expense of mesoderm. This observation suggests that Wnt3a regulates cell fate, acting as a switch between mesoderm and neural cell fates. T is a classic mouse mutation which, when homozygous, generates a paraxial mesoderm phenotype similar to the Wnt3a mutant phenotype. T disrupts the Brachyury gene, which encodes a highly conserved transcription factor expressed during gastrulation. We have shown that Brachyury is a direct transcriptional target of the Wnt3a, but not the Wnt5a, signaling pathway during the specification of mesoderm fates. Although several Wnts are coexpressed during gastrulation, it is clear from our functional studies that at least two of these Wnts have unique functions during gastrulation. This begs the question of how Wnt signaling specificity is achieved. In this regard, we have identified a number of genes that may function downstream of Wnts to select specific signal transduction pathways. One novel gene encodes a protein that interacts directly with the well-characterized Wnt signaling pathway component Dishevelled, and contains domains consistent with a role in linking Wnt signaling to cell polarity and the cytoskeleton. We are currently assessing the function of these novel proteins during mouse development. The transcriptional coactivator b-catenin is the primary effector of the canonical Wnt signaling pathway. Mutations in several components of the Wnt/b-catenin pathway have identified both oncogenes and tumor suppressors in this pathway that lead, in particular, to colorectal cancer. Embryos lacking Wnt5a display severe growth defects in the developing gastrointestinal tract. We are investigating whether aberrant activation of Wnt5a could lead to colorectal tumorigenesis. We are also assessing potential genetic interactions between Wnt5a and current mouse models of colorectal cancer. Ultimately, we hope to understand the role that Wnts play in the biology of intestinal stem cells.